The invention relates in particular to a method for testing the tightness of cavities by charging the cavity with a test gas.
In order to store natural gas or other technical gases in a larger scale, as for instance hydrogen or carbon dioxide for disposal, it is known to store the gas in underground located cavities, for instance in caverns mined of natural brine, aquifers or in other artificially created underground formations under pressure. These artificially created geological formations are pressure-tight after detailed examination and change their pressure-tight properties only in geological time periods. In order to be able to store gases in these cavities it is necessary to provide an access to the cavities by means of an access hole. This access hole consists of the hole itself and a piping, which forms a pipe shoe at the head of the cavity. The pipe shoe forms the transfer from the cavity to the artificially created access. It is a well-known technology to permanently seal such access holes.
In the following, the term “cavity” is to be understood as caverns, but also aquifers, created by human or naturally developed. Also if aquifers or porous rock layers are no cavities in the general linguistic usage, they still form underground and gas tight volumina. Such underground gas tight volumes are cavities according to the teachings of this patent application.
When creating the access hole to the cavity the cavity is in general filled with water or with natural brine. In order to test the tightness of the created access, the volume of the access, thus the piped access hole is charged with a test gas and the pressure is measured at the access after a predefined time, for instance 72 hours. If the pressure has changed only insignificantly, one can assume a tightness. A reason for the decrease of the pressure, which cannot be assigned to a missing tightness, is for instance the dissolving of the test gas in existing water or the natural brine still present in the cavity.
After the access has been finished and the tightness thereof is documented the cavity is emptied from the natural brine by a simultaneous filling with stored gas. Since the store cavities have a very large volume, the disposal of the water situated therein in case of closed cavities or the displacement of water in aquifers, thus porous rock layers, sometimes lasts multiple months. Once the cavity has reached an operation status, the cavity remains in this status until its closedown due to whatever reason. Since the properties of the cavities change only in geological time periods the operational suitability in human time periods depends only on the properties of the access hole. In order to document the suitability of an access hole provided for long-term operation subsequently within the frame of a routine investigation it would be thus necessary to document the tightness of the access hole itself. Furthermore, it is necessary to document in case of a planned pressure increase of the store cavity the access hole for its tightness in respect to the higher operational pressure of the cavity. A pressure measurement of a filled cavern conducted via time in acceptable time periods is however not possible due to the very large volumes of the gas storage. Artificially created caverns in geological salt formations are located in a depth of 300 m to 2500 m with a suitable height of up to 800 m and a diameter up to 250 m. Aquifer storages can have an even larger volume. Such large volumes do form in fact a Barostat. If stored gas escapes from these cavities in an already not any longer acceptable amount, the pressure changes almost imperceptibly in these very large cavities. It would be in principle possible for testing the tightness to detect a very small pressure drop over a longer time period. A pressure drop detected in this manner could, however, also be created by negligible temperature changes within the cavity via the longer observation period such that the pressure measurement is only a limited instrument for testing tightness.
An emptying of the cavity and filling with water or natural brine in order to document the tightness of the access hole as done when creating the access hole is almost not possible due to economic reasons. This is because the stored gas and the cushion gas existing in the dead volume has to be removed from the cavity and the cavity would have to be filled with water or natural brine. Such a process can last multiple months during which no gas can be stored. The disposal of the water after carrying out the measurement would be again a process lasting multiple months.